1. Field of the Invention
The present invention relates to an energy subtraction method and an energy subtraction apparatus, and in particular, relates to a technique for applying a method of acquiring energy subtraction images by taking at least two or more kinds of radiographic images at different radiation energy levels to radiography of various parts.
2. Description of the Related Art
Conventionally, there is an energy subtraction technology known as a kind of technology with which radiations with different energy distributions are applied to one and the same subject to shoot at least two or more kinds of radiographic images at different radiation energy levels, and then the radiographic images are taken as digital image signals to have respective pixels matched between the respective images and weighted to go through a subtraction process, whereby images exhibiting distributions of substances with particular radiation absorption characteristics can be acquired.
Particularly, a dual-energy subtraction technique using two images are well known in the art. With this technique, for example, it is possible to produce an image with bone image eliminated or soft tissue image eliminated from the imaged subject that includes both the bones and soft tissue.
Conventionally, accumulative phosphor sheets have been used in taking radiographic images. The accumulative phosphor sheet is accumulative phosphor that takes a form of a sheet, the accumulative phosphor being able to accumulate some of the radiation energy from the applied radiation, and later, in response to irradiation with excitation light such as visible light, or the like, exhibit photostimulable emission depending on the accumulated energy.
As a method of acquiring two radiographic images to be involved in a subtraction process in a case of performing the energy subtraction process using the accumulative phosphor sheets, there are two kinds which are generally known, one being a single-exposure scheme and the other being a two-exposure scheme.
In the single-exposure scheme, two accumulative phosphor sheets with a radiation separation filter such as a copper plate, or the like formed in between, or two accumulative phosphor sheets with different radiation absorption characteristics are stacked, and these two sheets are exposed at the same time with a single radiation irradiation, whereby both of the sheets will turn as though they have been irradiated with radiations with different radiation energy distributions in a single exposure.
In the two-exposure scheme, two accumulative phosphor sheets are used alternately for each exposure such that they are irradiated with radiations with two different radiation energy distributions (e.g. radiations at high energy and low energy), respectively, in two exposures.
Meanwhile, recently, a flat panel X-ray detector (FPD), where an enormous number of X-ray detecting elements using semiconductors, or the like, are arranged in a matrix in an X-ray detection surface, has come into use. With a single FPD, it is possible to take images as many times as needed. From a perspective that energy degree of freedom of radiation to apply is high, and good decomposed tissue images are easily obtainable particularly with the two-exposure scheme, the two-exposure scheme has come to attract more attention as the FPD has come to be used. Moreover, application of the energy subtraction technology to radiographies of abdominal and neck parts have been under consideration while the energy subtraction technology has already been recognized as effective in its application to the radiography of a chest part. Thus, the energy subtraction technology is expected to have extended range of application in the future.
With the two-exposure scheme, however, since occurrence of time lag between the exposures is inevitable, there is a problem in that artifacts can be generated due to movements of the subject between the exposures. In this respect, conventionally, various methods for reducing such artifacts in connection with the energy subtraction technology particularly in terms of radiography of a chest part with which possible influence of cardiac beat and breathing need to be taken into consideration have been proposed.
For example, there is a kind of a method that adopts a system for synchronizing acquisition of two images of a chest part of a patient with a cardiac cycle including: monitoring the cardiac cycle of the patient for detecting a first cardiac trigger; shooting the patient in response to a detection of the first cardiac trigger to form an X-ray image; monitoring the cardiac cycle of the patient for detecting a second cardiac trigger; recording an offset image; and combining the X-ray and offset images to acquire high quality X-ray image of the patient (cf. Japanese Patent Application Laid-Open No. 2002-325756, etc.). With the method using such system, possible artifacts that can be caused by cardiac motion of the patient are prevented from showing up on the X-ray images.
Furthermore, as an artifact reduction method which could be suitably applied to the energy subtraction process, for example, there is a method in which: shift vectors for respective control points arranged in a matrix are derived by template matching using localized areas of a low energy image in order to perform nonlinear transformation on the low energy image; the low energy image is transformed or distorted to be formed into an aligned low energy image; and the aligned low energy image is registered with a high energy image (cf. Japanese Patent Application Laid-Open No. 2003-244542, etc.). With such method, reduction of possible motion artifacts between the low and high energy images is attempted.